Magnetic lenses are widely used in the field of charged particle optics for focusing beams of charged particles, for example sub-atomic, atomic and polyatomic charged particles. The charged particles may be positively or negatively charged. Specific examples of charged particles for which magnetic lenses have been designed include ions, electrons, protons and other charged sub-atomic particles. Most commonly, magnetic lenses are used for focusing electrons.
It is known, for example, to use a magnetic lens to generate a charged particle image of a sample surface, such as an image of charged particles having a particular energy (which also means a particular range of energies) that have been ejected from the surface. If required, the image can be passed into an energy analyzer to produce an energy resolved image of the surface. The imaging field of the lens and the energy analyzer may be synchronously scanned to obtain an energy resolved image (e.g. a spectrum of particle energies for the image). From the energy resolved image it is possible to obtain information about the physical and chemical characteristics of a surface region of the sample.
Many different geometries of magnetic lens have been employed for different applications. One application of a magnetic lens is in photoelectron spectroscopy which is used to study the surface composition of a sample. In photoelectron spectroscopy, the sample is irradiated with electromagnetic radiation, typically from an X-ray or ultraviolet source, which causes electrons with energies of up to several thousand electron Volts (eV) to be emitted from the sample surface (so-called “photoelectrons”). The energies of the photoelectrons are characteristic of the elements and the chemical environment from which they were emitted. The two common forms of photoelectron spectroscopy are X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) where the names denote the type of radiation source used. The emitted photoelectrons are typically focused and separated according to their energy by an energy analyzer. The photoelectrons are then detected and a spectrum of photoelectron intensity against energy may be obtained, which can be used to yield information about the composition of the surface of the sample.
A magnetic lens may be used in XPS (in so-called imaging XPS) to focus photoelectrons emitted from a selected small area of the surface to thereby generate a photoelectron image of a sample surface. The photoelectron image is passed into an energy analyzer to produce an energy resolved image of the surface. Typically, the imaging field of the lens and the pass energy of the energy analyzer are synchronously scanned to create a spectrum of photoelectron energies for the image.
A conventional magnetic lens geometry for XPS is a so-called magnetic “snorkel” lens. The snorkel lens is mounted, in vacuum, below the sample as shown in FIG. 1A and described in more detail below. The snorkel lens geometry uses a central pole piece which extends into space from a surrounding annular pole piece or flux return. Such lenses are disclosed in EP 243 060 A, U.S. Pat. No. 5,286,974, U.S. Pat. No. 5,506,414, WO 99/53517 and JP 8-321272 A.
The advantage of a snorkel lens is that it allows good access to the sample from above (i.e. a clear line of sight), e.g. to facilitate surface processing and analysis, since the lens is completely positioned behind the sample. However, a snorkel lens has the drawback that the focal length of the lens is effectively fixed by its geometry so that little or no zoom of the magnification is possible. This therefore limits the ultimate resolution of the lens if it is also to be used for routine analysis with lower resolution. The fixed focal length of the lens also limits the thickness of a sample that can be used. In practice, some variation in the overall magnification of the XPS system can be achieved by varying the strength of subsequent lenses in the electron optical path but this fails to achieve the desired imaging resolution. In some applications, a much shorter focal length would be desirable, which leads to a smaller spherical aberration characteristic of the lens and a better imaging resolution.
In XPS applications, the snorkel lens is mounted inside an ultra high vacuum (UHV) chamber and so it is impractical to move or interchange lenses inside the vacuum chamber. Magnetic lens are also costly so that having several lenses would in any case add substantially to the cost of the system. A single snorkel lens could be designed with the shortest possible focal length but the working distance would then be so short that routine analysis would be severely hampered, e.g. for thicker samples, or where a large field of view is to be imaged.
Against the above background, the present invention has been made.